Recovery boilers for firing black liquor obtained through digesting of cellulose have been well-known for many decades. Their purpose is both to create energy through the firing and to recover chemicals used during the digesting of the cellulose and which are released in a melt condition during the firing and tapped from the bottom part of the recovery boiler. Recovery boilers are large plants and, besides a high amount of energy and recovered chemical substances, they also generate large quantities of fumes. These fumes contain inter alia nitrogen oxides arising from the firing of the liquor. These oxides come both from nitrogen containing materials in the liquor and from the nitrogen in the combustion air. The firing in a recovery boiler occurs at a relatively low temperature, about 1.200.degree. C., and it is presumed that the main part of the nitrogen oxides come from the black liquor.
In recent years continually stricter demands on the release of nitrogen oxides into the atmosphere have been introduced. It is commonly known that these oxides contribute to the acidifying of, and further unfavourable influence on, the nature. The contents of nitrogen oxides which are emitted from the recovery boilers of the forest industry are, however, low compared to those coming from cars, etc. Normally the contents are within 40-70 mg NO.sub.2 /MJ but these values will have to be lowered appreciably in the future.
The factors which influence the formation of nitrogen oxides are the amount of nitrogen in the fuel, how the nitrogen is bound, the speed of heating, the exposure time, the temperature and the content of oxygen. The most important parameter seems, however, to be the amount of oxygen supplied. By lowering the excess of air and thereby lowering the content of oxygen in the firing area, it is possible to lower the formation of nitrogen oxides. At the same time as the oxygen content sinks to a level perhaps less than 2-3 percent by volume, the contents of carbon monoxide will however increase to very high values due to the incomplete firing of carbon. It has therefore previously been proposed to maintain a low content of oxygen in the lower part of the recovery boiler for formation of a small amount of nitrogen oxides and to combine this low oxygen content with a final firing of the remaining carbon monoxide in an excess air register at a higher level in the recovery boiler. It is important that the carbon monoxide be oxidised to carbon dioxide for obtaining a satisfactory energy development, but at this latter combustion the content of nitrogen oxides also increases.
It has also been proposed to add reducing gas such as ammonia or city gas above the liquor input region to obtain a reduction of thus formed nitrogen oxides. Air has thereafter been added for final combustion of the components in the gases which can be burned to thereby develop further energy. Despite all these attempts to perform the firing in such a way that low contents of nitrogen oxides are created, a satisfactory combustion method has not yet been obtained. The contents of nitrogen oxides are still too large.